The U.S. patent of Nguyen, U.S. Pat. No. 5,155,302, granted Oct. 13, 1992, hereby incorporated herein by reference, describes the problem of making electrical connections to each of a matrix array of contact pads or bonding pads on one side of a microelectronic chip. For example, photodetector arrays require electrical contact to each of hundreds or thousands of photodetectors arranged as a matrix array on a single chip. The solution provided by the Nguyen patent is to superimpose over the contact array a number of layers of anisotropic material and insulator material having conductive vias. This allows interconnection to the various individual contacts by interconnection to peripheral contacts arranged on each successive insulator layer of the package. Electrical contact can be made by wire bonding to each one of the peripheral arrangement of bonding pads, which provides interconnection to all of the contacts of the matrix array.
An interconnection method that is, in many respects, superior to wire bonding is known as the surface mount method. By this method, solder elements are first bonded to bonding pads of a chip. The chip is next placed over a substrate such that the solder elements contact bonding pads of the substrate. The solder elements are then melted or reflowed so that each solder element forms an intimate bond with the bonding pad beneath it.
As described, for example, in the paper, "Cost-Effective Interconnections," Surface Mount Technology, July 1993, pp. 18-20, hereby incorporated herein by reference, a particular form of the surface mount method known as the ball grid array method can be used to connect simultaneously the solder balls of a matrix array to matching bonding pads of a substrate. A solder element or solder ball is attached to each bonding pad of a matrix array of bonding pads on a chip, with all of the solder balls being simultaneously reflowed and attached to a matrix array of bonding pads on the substrate. In this manner, hundreds of solder joints can be made simultaneously, with the simultaneous reflow causing a settling of the balls such that, for example, solder balls that are undersized may nevertheless provide good contact. The ball grid array method therefore provides known advantages of both efficiency and reliability.
Where the density of the solder balls is extremely great, such as in photodetector arrays, there still may be a problem with making contact to the matrix array of bonding pads to which the solder balls are bonded; the space between the bonding pads may be insufficient to provide access conductors at reasonable cost. There is no obvious way of using the techniques of the Nguyen patent while still obtaining the advantages of the ball grid array method. There is therefore a long-felt need in the industry for methods for making separate electrical contact to each bonding pad of a dense matrix array of the type used for ball grid array bonding.